Precipitated magnesium carbonate

ABSTRACT

The present invention relates to a process for preparing hydromagnesite in an aqueous environment. The invention further relates to such hydromagnesite having a specific platy-like morphology in combination with a specific average particle size and to their use as minerals, fillers and pigments in the paper, paint, rubber and plastics industries and to the use as flame-retardant.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is U.S. National phase of PCT Application No.PCT/EP2010/066664, filed Nov. 2, 2010, which claims priority to EuropeanApplication No. 09174954.9, filed Nov. 3, 2009 and U.S. ProvisionalApplication No. 61/280,918, filed Nov. 10, 2009.

The present invention relates to a process for preparing hydromagnesitein an aqueous environment. The invention further relates to suchhydromagnesite having a platy-like morphology in combination with aspecific average particle size and to their use as minerals, fillers andpigments in the paper, paint, rubber and plastics industries and to theuse as flame-retardant.

Hydromagnesite or basic magnesium carbonate, which is the standardindustrial name for hydromagnesite, is a naturally occurring mineralwhich is found in magnesium rich minerals such as serpentine and alteredmagnesium rich igneous rocks, but also as an alteration product ofbrucite in periclase marbles. Hydromagnesite is described as having thefollowing formula:Mg₅(CO₃)₄(OH)₂.4H₂O

It should be appreciated that hydromagnesite is a very specific mineralform of magnesium carbonate and occurs naturally as small needle-likecrystals or crusts of acicular or bladed crystals. In addition thereto,it should be noted that hydromagnesite is a distinct and unique form ofmagnesium carbonate and is chemically, physically and structurallydifferent from other forms of magnesium carbonate. Hydromagnesite canreadily be distinguished from other magnesium carbonates by x-raydiffraction analysis, thermogravimetric analysis or elemental analysis.Unless specifically described as hydromagnesite, all other forms ofmagnesium carbonates (e.g. artinite (Mg₂(CO₃)(OH)₂.3H₂O), dypingite(Mg₅(CO₃)₄(OH)₂.5H₂O), giorgiosite (Mg₅(CO₃)₄(OH)₂.5H₂O), pokrovskite(Mg₂(CO₃)(OH)₂.0.5H₂O), magnesite (MgCO₃), barringtonite (MgCO₃.2H₂O),lansfordite (MgCO₃.5H₂O) and nesquehonite (MgCO₃.3H₂O)) are nothydromagnesite within the meaning of the present invention and do notcorrespond chemically to the formula described above.

Besides the natural hydromagnesite, synthetic hydromagnesites (orprecipitated magnesium carbonates) can be prepared. For instance, U.S.Pat. No. 1,361,324, U.S. Pat. No. 935,418, GB 548,197 and GB 544,907generally describe the formation of aqueous solutions of magnesiumbicarbonate (typically described as “Mg(HCO₃)₂”), which is thentransformed by the action of a base, e.g., magnesium hydroxide, to formhydromagnesite. Other processes described in the art suggest to preparecompositions containing both, hydromagnesite and magnesium hydroxide,wherein magnesium hydroxide is mixed with water to form a suspensionwhich is further contacted with carbon dioxide and an aqueous basicsolution to form the corresponding mixture; cf. for example U.S. Pat.No. 5,979,461.

Additionally, general processes for preparing magnesium carbonate aredescribed in the art. For example, EP 0 526 121 describes acalcium-magnesium carbonate composite consisting of calcium carbonateand magnesium carbonate hydroxide and a method for the preparationthereof. Furthermore, GB 594,262 relates to a method and apparatus fortreating magnesia-containing materials, such as magnesium and calciumcarbonate materials for obtaining respective carbonates in discrete andseparate forms, by controlled carbonation such that the magnesium andcalcium carbonates may be separated by mechanical means and withattainment of special utilities in separated products. US 2007194276describes a method of reductively bleaching a mineral slurry comprisingadding in the mineral slurry an effective amount of a formamidinesulfinic acid (FAS) and an effective amount of a borohydride toreductively bleach the mineral slurry.

In practice, hydromagnesite is used in huge quantities in the paper,rubber and plastics industries for various purposes such as coatings,fillers, extenders and pigments for papermaking as well asflame-retardants in electrical wires and cables but also to impartresistance to chemicals in fibers. For example, EP 0 543 262, EP 0 393813, JP 21 50 436, JP 22 55 843, JP 51 70 984, JP 50 98 085 and KR2003/0040953 describe flame-retardant compositions comprisinghydromagnesite in admixture with other magnesium compounds such ashuntite, dolomite and/or magnesium hydroxide. In this context,hydromagnesite in combination with various magnesium compounds isusually added into a resin composition for providing flame resistanceand high mechanical strength so that such compositions can be used as acovering or insulation material for electric wires or cables, flamearresting materials, wall materials for various areas such as theautomotive sector, for the production of housings for electricalappliances or in the building sector.

Another application for hydromagnesite is described in WO 2009/008600which relates to a spandex fiber containing hydromagnesite and havingresistance to chlorine without affecting intrinsic properties of thepolyurethane polymer. Furthermore, WO 97/09473 describes spandexcontaining particles of a mineral mixture of huntite and hydromagnesite,wherein the spandex is described as having decreased tackiness andincreased resistance to chlorine-induced degradation.

Additionally, hydromagnesite in combination with other magnesiumcompounds is used in the paper industries in order to impartprintability, a high brightness at high opacity but also suitablesmoothness and gloss to paper products such as magazines. In thisrespect, JP 2003/293291 describes coated paper produced by disposing anadhesive and a coating layer consisting mainly of at least one ofhuntite and hydromagnesite on base paper, wherein the resulting coatedpaper has high brightness, a high surface-masking effect and excellentprinting suitability.

Hydromagnesite and other magnesium compounds, e.g. magnesium carbonateand magnesium hydroxide, can also be incorporated as a filler inwrapping papers of smoking articles such as cigarettes or cigars inorder to control many physical properties or characteristics such as thetar delivery per puff, burn rate, puff count, etc. One particularlyimportant aspect of a smoking article that can be controlled by suchwrapping paper is the sidestream smoke, which is the smoke given off bythe burning end of a smoking article between puffs. However, as suchsmoke may be objectionable to other people near the smoker severalattempts have been made to reduce such sidestream smoke through the useof various magnesium compounds. For example, U.S. Pat. No. 5,092,306relates to a smoking article wrapper, and in particular, cigarette paperwhich uses magnesite as a filler composition. Others have used physicalmixtures of magnesium hydroxide and hydromagnesite, e.g. U.S. Pat. No.5,927,288 and U.S. Pat. No. 5,979,461, while others have made attemptsfor developing compositions wherein the amount of magnesium hydroxide isreduced by replacing this hydroxide with other magnesium compounds. Forexample, U.S. Pat. No. 5,253,660 discloses a cigarette or cigar wrapperwherein the paper filler consists of two homogeneously intermixedminerals, namely huntite and hydromagnesite, alone, or admixed withcalcium carbonate or magnesium hydroxide or calcium carbonate andmagnesium hydroxide and carbon.

However, with respect to the aforementioned possible applications ofhydromagnesite, it is to be noted that there are significant constrainsregarding the suitability of the corresponding filler particles or theirapplication field. Hydromagnesite obtained from natural sources orprepared by processes described in the prior art for use as filler orcoatings in paper applications, in smoking articles and/or as flameretardant usually has an average particle size of about 5 μm or more. Inthis context, it is to be noted that the thickness of, for example,wrapping papers for smoking articles is generally in the range of about30 μm, so that the incorporation of hydromagnesite into smoking articlesoften do not impart the desired properties such as smoothness onto thesurface of such articles and, thus, the physical and optical propertiesof the obtained products are not always satisfactory. Additionally, aswell known from kaolin and its use as surface coating and filler in thefield of paper applications, the morphology of the particles plays alsoa decisive role for imparting the desired optical and physicalproperties such as good printability, high brightness at high opacity,moderate porosity and a favorable smoothness and gloss into paperproducts such as magazines. For many applications, a platy-likemorphology of the particles is highly favorable for obtaining saidproperties. The provision of platy-like particles having a smallparticle size would be especially advantageous. In this context, it isfurther to be noted that a mechanical comminution of particles isusually not a suitable and effective method for obtaining smallerparticles of hydromagnesite.

Thus, there is still a need in the art for providing alternativeprocesses for preparing hydromagnesite, wherein such process should besimple and inexpensive and should provide the possibility of controllingparticular parameters such as the particle size in combination with themorphology and the density of the obtained particles.

Accordingly, it is an objective of the present invention to provide analternative process for preparing hydromagnesite preferablyhydromagnesite having a specific platy-like morphology in combinationwith decreased particle sizes. Another objective of the presentinvention may be seen in the provision of a process for preparinghydromagnesite having a high absolute density. A further objective ofthe present invention may be seen in the provision of a process forpreparing hydromagnesite having improved optical properties andespecially a high degree of whiteness R457. Even a further objective ofthe present invention may be seen in the provision of a process whichcan be carried out in a simple way. A still further objective of thepresent invention may be seen in the provision of a process which can becarried out under mild conditions and the obtained hydromagnesite can beused directly without further complex and costly processing steps. Evenanother objective of the invention may be seen in the provision of aprocess, in which hydromagnesite material can be prepared in high yield.Further objects can be gathered from the following description of theinvention.

In order to fulfill the foregoing need(s) a process according to thesubject-matter as defined herein in claim 1 is provided.

Advantageous embodiments of the inventive method are defined in thecorresponding sub-claims and the specification.

According to one aspect of the present application a process forpreparing hydromagnesite in an aqueous environment is provided, theprocess comprising the steps of:

a) providing at least one magnesium oxide source;

b) providing gaseous CO₂ and/or carbonate-comprising anions;

c) slaking of said magnesium oxide source of step a) to convert themagnesium oxide at least partially into magnesium hydroxide;

d) contacting the obtained magnesium hydroxide of step c) with saidgaseous CO₂ and/or carbonate-comprising anions of step b) to convert themagnesium hydroxide at least partially into precipitated nesquehonite;and

e) treating the obtained precipitated nesquehonite of step d) in aheat-ageing step.

The inventors surprisingly found that the foregoing process allows forthe efficient and controlled production of hydromagnesite. According tothe process of the present invention hydromagnesite having a platy-likemorphology as well as decreased particle sizes can be provided orprepared directly. More precisely, the inventors found that themorphology as well as the physical values of hydromagnesite beingobtained by said process can be improved by specifically controlling oradjusting the process conditions during the preparation of saidhydromagnesite. The process involves slaking a magnesium oxide source,like “pure” magnesium oxide, a magnesium oxide containing mineral oranother source containing magnesium compounds such as dolomite, whichcan be used for preparing the magnesium oxide. The resulting magnesiumhydroxide in a further process step undergoes a reaction with gaseouscarbon dioxide and/or carbonate-comprising anions resulting inprecipitated nesquehonite as an intermediate product. The carbonizationtemperature according to one embodiment of the invention should becontrolled and preferably should be below 35° C. Finally, hydromagnesiteis directly obtained after transforming said precipitated intermediateproduct by a heat treatment step. According to one embodiment of theinvention, the thus obtained precipitated intermediate product is groundprior to further processing. The hydromagnesite obtained by theinventive process provides several advantageous characteristics, likesmall particle size in combination with a platy-like morphology and highabsolute density.

It should be understood that for the purposes of the present invention,the following terms have the following meanings:

“Hydromagnesite”, “basic magnesium carbonate” or “magnesium carbonatehydroxide” in the meaning of the present invention defines asynthetically prepared material of magnesium carbonate with the chemicalformula Mg₅(CO₃)₄(OH)₂.4H₂O.

“Nesquehonite” in the meaning of the present invention defines asynthetically prepared material of magnesium carbonate with the chemicalformula MgCO₃.3H₂O or Mg(HCO₃)(OH).2H₂O.

The term “precipitation” in the meaning of the present invention refersto the formation of a solid material in a solution during a chemicalreaction.

A “suspension” or “slurry” in the meaning of the present inventioncomprises insoluble solids and water and optionally further additivesand usually contains large amounts of solids and, thus, is more viscousand generally of higher density than the liquid from which it is formed.

The term “slaking” or “slake” in the meaning of the present inventionrefers to the hydration of magnesium oxide by contacting said compoundswith water or moisture.

The term “calcining” in the meaning of the present invention refers to athermal treatment process applied to solid materials causing loss ofmoisture, reduction or oxidation, and the decomposition of carbonatesand other compounds resulting in an oxide of the corresponding solidmaterial.

The term “carbonation” in the meaning of the present invention refers toa process in which at least one hydroxide group is replaced bycarbonate.

The term “heat-ageing” in the meaning of the present invention relatesto a thermal treatment process in which crystals having initially ahigher internal energy state undergo a phase transformation bydissolving and redepositing into crystals having a lower internal energystate.

According to another aspect of the present invention, hydromagnesite isprovided, wherein said hydromagnesite is obtainable by the inventiveprocess for preparing hydromagnesite. The hydromagnesite preferably hasa platy-like morphology in combination with a decreased particle sizeand improved physical and optical properties. According to a furtheraspect, said hydromagnesite obtainable by the process of the presentinvention has an increased density. According to another aspect, thepresent invention refers to the use of said hydromagnesite as mineral,filler and pigment in paper, paint, rubber and plastics applications andto its use as flame-retardant.

According to one preferred embodiment of the present invention, the atleast one magnesium oxide source comprises magnesium oxide, magnesite,dolomite, huntite, magnesium carbonate, magnesium hydroxide, brucite ormixtures thereof.

According to another preferred embodiment of the inventive process, thegaseous CO₂ comes from an external CO₂ supply or from the recirculationof CO₂ or both.

According to yet another preferred embodiment of the inventive process,the carbonate-comprising anions are selected from the group consistingof sodium carbonate, potassium carbonate, sodium hydrogen carbonate,potassium hydrogen carbonate or mixtures thereof.

According to one preferred embodiment of the inventive process, thestarting temperature of step d) is adjusted to a temperature of between5° C. and 35° C. and most preferably to a temperature of between 10° C.and 30° C.

According to another preferred embodiment of the present invention, theheat-ageing step of step e) is carried out at a temperature of at least90° C., preferably in the range between 90° C. and 150° C., morepreferably at a temperature of between 110° C. and 140° C., even morepreferably at a temperature of between 120° C. to 135° C. and mostpreferably at a temperature of about 130° C.

According to yet another preferred embodiment of the present invention,the heat-ageing step is carried out for a period of time of 20 min to 60min, preferably for a period of time of 20 min to 40 min and mostpreferably for a period of time of 25 to 35 min.

According to one preferred embodiment of the present invention, theprecipitated nesquehonite obtained in step d) is ground prior to theheat-ageing step of step e).

According to another preferred embodiment of the present invention, theprecipitated nesquehonite obtainable after the grinding comprisesparticles of which at least 50% by weight have an an average particlesize of less than 25 μm, more preferably of less than 20 μm, even morepreferably of less than 15 μm and most preferably of less than 10 μm.

According to yet another preferred embodiment of the present invention,the hydromagnesite obtainable by the process has a specific BET surfacearea of 10 m²/g to 150 m²/g, more preferably of 10 m²/g to 100 m²/g andmost preferably of 20 m²/g to 70 m²/g.

According to one preferred embodiment of the present invention, thehydromagnesite obtainable by the process has a degree of whiteness R457of at least 80%, more preferably of at least 85%, even more preferablyof between 85% and 99% and most preferably of between 85% and 99%.

According to another preferred embodiment of the present invention, thehydromagnesite obtained by the process comprises particles having anaverage particle size d₅₀ of less than 20 μm, preferably of less than 15μm, more preferably of less than 10, and most preferably of less than 5μm.

According to yet another preferred embodiment of the present invention,the hydromagnesite obtained by the process is further treated with fattyacids, preferably selected from the group consisting of one or morefatty acids selected from stearic acid, palmitic acid, behenic acid,montanic acid, capric acid, lauric acid, myristic acid, isostearic acidand cerotic acid.

FIG. 1 shows a simplified flow-chart illustrating the process forpreparing hydromagnesite according to the process of the presentinvention.

As set out above, the inventive process for preparing hydromagnesitehaving improved optical and physical properties comprises the steps a),b), c), d) and e). In the following, it is referred to further detailsof the present invention and especially the foregoing steps of theinventive process for preparing magnesium carbonate suspensions having aspecific morphology in combination with decreased particle size as wellas excellent optical and physical properties. Those skilled in the artwill understand that many embodiments described herein can be combinedor applied together.

Characterization of Step a): Provision of a Magnesium Oxide Source

According to step a) of the process of the present invention, at leastone magnesium oxide source is provided.

The at least one magnesium oxide source in the meaning of the presentinvention refers to

magnesium oxide; and/or

a material in which magnesium oxide naturally occurs; and/or

a material in which at least one magnesium compound occurs which can beconverted into magnesium oxide.

Accordingly, the at least one magnesium oxide source is preferablyselected from magnesium oxide, magnesium oxide containing minerals,magnesium containing materials and mixtures thereof. Preferred magnesiumoxide sources are magnesite and dolomite, for which a calcining step isneeded to convert the contained magnesium compounds into the desiredmagnesium oxide.

In the case where the at least one magnesium oxide source is selectedfrom magnesium oxide, said magnesium oxide is preferably in the form ofa powder having a magnesium oxide content of more than 95 wt.-% and morepreferably of more than 98 wt.-%, based on the weight of the powder. Ina preferred embodiment, the particles of magnesium oxide in the powderare of small particle sizes; i.e. the particles of magnesium oxide havean average particle size d₉₉ value of less than 100 μm and a d₉₅ valueof less than 75 μm, as determined by laser diffraction using theinstrument CILAS 920 particle-size-analyzer of CILAS, Orleans, France.

A material in which a form of magnesium oxide naturally occurs isunderstood to be a magnesium oxide containing mineral. An example ofsuch magnesium oxide rich mineral is represented for example bypericlase, which occurs naturally in contact metamorphic rocks and is amajor component of most basic refractory bricks.

By contrast, a material from which magnesium oxide can be syntheticallyobtained, i.e. can be converted into magnesium oxide may be anymagnesium containing material, for example, materials comprisingmagnesium hydroxide and/or magnesium carbonate. If the at least onemagnesium oxide source is a magnesium containing material, said materialcomprises the magnesium compound(s) preferably in an amount of at least15 wt.-%, more preferably of at least 25 wt.-% and most preferably of atleast 40 wt.-% based on the total weight of the magnesium containingmaterial.

In the case where the at least one magnesium oxide source is a magnesiumcontaining material, said material is preferably selected from magnesiumcarbonate, magnesium hydroxide, magnesite, brucite, dolomite, huntite,magnesium chloride rich brine, seawater from which magnesium oxide canbe obtained and mixtures thereof. In this context, the term “magnesiumcarbonate” according to the present invention comprises anhydrousmagnesium carbonate as well as forms of magnesium carbonate comprisingcrystal water (hydrate).

According to the present invention, the magnesium oxide source of stepa) of the process of the present invention is not restricted to thespecific minerals and/or materials described above. Rather any mineraland/or material can be used provided that said mineral and/or materialcomprises a sufficient amount of magnesium oxide and/or a correspondingmagnesium oxide containing mineral and/or a material or mineral whichcan be at least partially converted into magnesium oxide.

Said at least partial conversion of a magnesium compound into magnesiumoxide is preferably carried out by calcining said materials. Saidcalcining step can be carried out by any conventional calcining processknown to the skilled person.

In one preferred embodiment, the at least one magnesium oxide source ismagnesite (MgCO₃), which is a frequently occurring mineral merelyexclusively consisting of anhydrous magnesium carbonate. Such materialin the meaning of the present invention would be regarded as magnesiumcontaining material. Said magnesite is preferably calcined attemperatures of about 600° C. to 900° C. in order to obtain magnesiumoxide in the form of caustic calcined magnesite. The obtained particlesof caustic calcined magnesite are characterized by their high porosityand possess a high reactivity due to their large inner surface and,thus, are especially suitable for the purposes of the present invention.Alternatively or additionally, other forms of magnesium carbonate, forexample, synthetically prepared magnesium carbonate in the anhydrousform and/or forms comprising crystal water and/or other naturallyoccurring forms of magnesium carbonate may also be used for providingthe at least one magnesium oxide source. The magnesium carbonates mayalso be calcined under the same conditions as applied for magnesite inorder to convert these magnesium carbonates at least partially intomagnesium oxide.

Preferably, the obtained caustic calcined magnesite has a content ofmagnesium oxide of more than 85 wt.-%, more preferably more than 90wt.-% and most preferably of more than 92 wt.-%, based on the totalweight of the caustic calcined magnesite. In a preferred embodiment,said caustic calcined magnesite has a content of magnesium oxide ofbetween 90 wt.-% and 99 wt.-% and most preferably of between 92 wt.-%and 98 wt.-%, based on the total weight of the caustic calcinedmagnesite. Magnesium oxide obtained from other magnesium carbonates haspreferably a content of magnesium oxide of more than 90 wt.-%, forexample in the range between 95 wt.-% and 99 wt.-%, based on the totalweight of the magnesium oxide.

The particles of the caustic calcined magnesite and/or magnesium oxideobtained from other forms of magnesium carbonate preferably have aparticle size distribution as conventionally employed for thematerial(s) involved in a corresponding process for converting amagnesium oxide source at least partially into magnesium hydroxide asused in step c) of the process of the present invention. In general, thecaustic calcined magnesite particles and/or particles of magnesium oxideobtained from other forms of magnesium carbonate have an averageparticle size d₅₀ value of from 1 μm to 100 μm, preferably from 5 μm to50 μm and most preferably from 10 μm to 25 μm, for example between 15 μmand 20 μm, as determined by laser diffraction using the instrument CILAS920 particle-size-analyzer of CILAS, Orleans, France.

As used herein and as generally defined in the art, the d₅₀ value isdefined as the size at which 50% (the median point) of the particle massis accounted for by particles having a particle size equal to thespecified value.

In the case where the magnesium containing material is magnesiumhydroxide or a material comprising a high magnesium hydroxide content,said material may be directly subjected to step d) of the process of thepresent invention. Alternatively or additionally, said magnesiumcontaining material may be first subjected to the slaking of step c) ofthe process of the present invention.

In another preferred embodiment, the at least one magnesium oxide sourceis dolomite which is merely composed of calcium magnesium carbonate(CaMg(CO₃)₂) and thus representing a magnesium containing mineral. Forthe purposes of the present invention, any available variety of dolomitecan be used as the at least one magnesium oxide source. However, in onepreferred embodiment, the dolomite is white dolomite representing arelatively pure dolomite which may be e.g. obtained or extracted fromNorwegian Talc's Hammerfall deposits. In order to convert the magnesiumcompound, i.e. the carbonates, contained in said dolomite at leastpartially into magnesium oxide, the dolomite is preferably calcined athigh temperatures, wherein said calcining step can be carried out by anyconventional calcining process known to the skilled person.

In one preferred embodiment, the dolomite is calcined at temperatures ofbetween 900° C. and 1200° C. and more preferably at a temperature ofbetween 1000° C. and 1100° C. in order to obtain burned dolomite(MgO.CaO). Alternatively, the dolomite is calcined at temperatures ofbetween 600° C. and 900° C., more preferably of between 600° C. and 800°C. and most preferably at a temperature of about 750° C. in order toobtain half-burned dolomite (MgO.CaCO₃). The chemical characteristicssuch as reactivity of the half-burned or burned dolomite, i.e. of theobtained magnesium oxide, depend mainly on the temperatures andcalcining processes used which are well known to the skilled person. Forinstance, if the dolomite is calcined at a temperature in the range of1000° C. to 1100° C. in order to obtain burned dolomite, saidtemperature is preferably maintained for a period of time of between 30min to 120 min and most preferably for a period of time of between 45min and 90 min, for example for about 60 min.

The dolomite subjected to the calcining process has preferably a sizedistribution as conventionally employed for the material(s) involved ina corresponding calcining process for obtaining burned or half-burneddolomite. In this context, it is to be noted that dolomite as well asother magnesium containing materials such as for example magnesite usedas the at least one magnesium oxide source according to the presentinvention are usually in the form of rocks and grains of various sizes.For obtaining a sufficient amount of magnesium oxide having a sufficienthigh reactivity and/or a sufficient specific surface area, said rocksand/or grains are preferably comminuted by a mechanical processing stepprior to calcining said magnesium rich materials resulting in areduction of the original grain size. As a result of the comminution,the mean grain size of the obtained particles are preferably in therange of 1 mm to 250 mm, preferably in the range of 1 mm to 150 mm, evenmore preferably in the range of 1 mm to 100 mm and most preferably inthe range of 1 mm to 75 mm. In one preferred embodiment, said particleshave preferably a mean grain size in the range between 10 mm and 50 mm,as measured by screening on analytical test sieves from Retsch®, Germanyand determining mass fractions of selected size ranges.

Such comminution steps may, for example, be performed under conditionssuch that refinement predominantly results from impacts with a secondarybody, i.e. in one or more of a vertical bead mill, a ball mill, a rodmill, a vibrating mill, a roll crusher, a centrifugal impact mill, anattrition mill, a pin mill, a hammer mill, a pulveriser, a shredder, ade-clumper, a knife cutter, or other such equipment known to the skilledperson, or may be performed under conditions such that autogenousgrinding takes place. In one preferred embodiment, such comminution stepis carried out by grinding rocks and grains of, for example, dolomite byusing a ball mill.

In the case where magnesium containing materials have been calcined andare subsequently used as the corresponding magnesium oxide source instep c) of the process of the present invention, said step ofcomminution may, alternatively or additionally, be carried out aftersaid magnesium containing material has been calcined, i.e. prior to stepc) of the process of the present invention. If magnesium oxide and/ormagnesium oxide containing minerals are used as the at least onemagnesium oxide source, such step of comminution may be also performedprior to step c) of the process of the present invention. Suchcomminution prior to step c) of the process of the present invention ispreferably carried out if there is a particle size distribution of themagnesium oxide source which is considered too broad and/or if themedian diameter of the magnesium oxide source is above 150 μm.Accordingly, the particles of the magnesium oxide source used in step c)of the process of the present invention are preferably of an averageparticle size d₅₀ value of from 1 μm to 150 μm, more preferably from 5μm to 100 μm and most preferably from 10 μm to 75 μm, as measured bylaser diffraction using the instrument CILAS 920 particle-size-analyzerof CILAS, Orleans, France

Characterization of Step b): Providing Gaseous CO₂ and/orCarbonate-Comprising Anions

According to step b) of the process of the present invention, gaseousCO₂ and/or carbonate-comprising anions are provided.

The carbon dioxide provided in step b) may be any form of gaseous CO₂,for example, carbon dioxide, a carbon dioxide containing gas or carbonicacid, i.e. carbon dioxide dissolved in water.

In one preferred embodiment, the gaseous CO₂ is derived from the gaseouseffluent obtained by calcining various magnesium oxide sources providedin step a) of the same process, i.e. the gaseous carbon dioxide issupplied by the recirculation of CO₂. By calcining of such sources suchas magnesium carbonate, for example, in the form of magnesite, a risingtemperature during the calcining process increases the amount of carbondioxide gas released during the process. Preferably, the generatedcarbon dioxide is vented out of the reaction vessel as the obtainedmagnesium oxide has an affinity to absorb moisture as well as carbondioxide, so that said compound undergoes the reaction back to themagnesium carbonate. Such gases contain approximately 5% to 40% byvolume of CO₂ and are preferably used after purification and, optionallyup-concentration or dilution of the gaseous effluent.

Additionally or alternatively, the carbon dioxide can be supplied froman external source such as from a steel cylinder or from flue gasesand/or exhaust gases of industrial processes using furnaces and kilnsand/or from suitable reactions of carbonate salts with acids etc.However, it is to be noted that the carbon dioxide source provided instep b) of the inventive process is not particularly limited providedsaid source contains no reactive gas.

The gaseous carbon dioxide of step b) may be provided in a concentratedform or in a diluted form. If the gaseous carbon dioxide is provided ina diluted form, the carbon dioxide is preferably provided in admixturewith air or the like.

In this case, the gaseous carbon dioxide provided in step b) of theprocess of the present invention has, in terms of volume, aconcentration in, for example, air of less than 40 vol.-%, morepreferably less than 35 vol.-% and most preferably between 10 vol.-% and30 vol.-% based on the total volume of the gaseous composition. Theminimum carbon dioxide content in the carbon dioxide source may be about8 vol.-%, based on the total volume of the gaseous composition.

Additionally or alternatively, carbonate-comprising anions are providedin step b) of the process of the present invention. Thecarbonate-comprising anions of step b) may be provided in any form ofcarbonate salts which are soluble in water, i.e. dissolve in water toform a homogeneous solution. In one preferred embodiment,carbonate-comprising anions refer to carbonate salts, which when mixedwith deionised water provide a solubility of more than 50 g/l at 20° C.,preferably of more than 100 g/l at 20° C., more preferably of more than150 g/l at 20° C. and most preferably of more than 200 g/l at 20° C.

Accordingly, the carbonate-comprising anions of step b) are preferablyselected from the group comprising alkali carbonates and/or alkalihydrogen carbonates, wherein the alkali ion of the alkali carbonateand/or alkali hydrogen carbonate is selected from sodium, potassium andmixtures thereof. Sodium carbonate, potassium carbonate, sodium hydrogencarbonate, potassium hydrogen carbonate and mixtures thereof arepreferred carbonate-comprising anions of step b) of the process of thepresent invention. In one preferred embodiment, the carbonate-comprisinganions are sodium carbonate.

In the context of the present invention, the term “sodium carbonate”shall include sodium carbonate in the anhydrous form as well as formscomprising crystal water (hydrate). In one preferred embodiment, thesodium carbonate of the present invention is anhydrous sodium carbonate(Na₂CO₃) or sodium carbonate decahydrate (Na₂CO₃.10H₂O).

The term “potassium carbonate” also refers to potassium carbonate in theanhydrous form as well as forms comprising crystal water (hydrate).Preferably, the potassium carbonate of the present invention isanhydrous potassium carbonate (K₂CO₃).

In the context of the present invention, the term “sodium hydrogencarbonate” shall include sodium hydrogen carbonate in the anhydrous formas well as forms comprising crystal water (hydrate). Preferably, thesodium hydrogen carbonate of the present invention is anhydrous sodiumhydrogen carbonate (NaHCO₃).

In the context of the present invention, the term “potassium hydrogencarbonate” shall also include potassium hydrogen carbonate in theanhydrous form as well as forms comprising crystal water (hydrate).Preferably, the potassium hydrogen of the present invention is anhydrouspotassium hydrogen carbonate (KHCO₃).

In case carbonate-comprising anions are provided in step b) of theprocess of the present invention, said carbonate-comprising anions canbe provided in any appropriate solid form, e.g. in the form of granulesor a powder. Alternatively, said carbonate-comprising anions can beprovided in the form of a suspension or solution.

Characterization of Step c): Slaking of Said Magnesium Oxide Source ofStep a)

According to step c) of the process of the present invention, said atleast one magnesium oxide source of step a) is slaked to convert atleast a part of the magnesium oxide into magnesium hydroxide.

“Slaking” in the meaning of the present invention refers to a process inwhich magnesium oxide is hydrated. Thus, the term “slaking” refers to aprocess of adding water to magnesium oxide to produce magnesiumhydroxide. Accordingly, the particles of magnesium oxide of the at leastone magnesium oxide source provided in step a) are hydrated in a slakingprocess, which is carried out by contacting the magnesium oxide of theat least one magnesium oxide source with water. According to theinventive process the magnesium oxide is at least partially convertedinto magnesium hydroxide.

The water to be used in the slaking process of step c) may be any wateravailable such as tap water and/or deionised water. Preferably, thewater used for slaking the at least one magnesium oxide source of stepa) is tap water.

In one preferred embodiment of the present invention, the at least onemagnesium oxide source of step a) is added to the water in one portionand/or continuously over a period of 1 h or less, preferably over aperiod of 45 min or less, more preferably over a period of 30 min orless and most preferably over a period of 15 min or less to yield orprovide a suitable solid content in the resulting suspension. In anotherpreferred embodiment, the at least one magnesium oxide source of step a)is added to water over a period of 10 min or less to a suitable solidcontent in the resulting suspension. In a further preferred embodimentof the present invention, the at least one magnesium oxide source ofstep a) is added to the water in several portions to a suitable solidcontent in the resulting suspension, preferably in two to five portions,more preferably in two to four portions, even more preferably in two tothree portions and most preferably in two portions.

In the case where the at least one magnesium oxide source of step a) isadded to the water in several portions, the at least one magnesium oxidesource of step a) is preferably added in about equal portions to thewater. As an alternative, it is also possible to add the at least onemagnesium oxide source of step a) in unequal portions to the water, i.e.in larger and smaller portions. In one preferred embodiment, the largerportion is added first followed by the addition of the smaller portionof the at least one magnesium oxide source to the resulting suspensionin order to slake or hydrate the at least one magnesium oxide sourceprovided in step a). In another preferred embodiment, the smallerportion of the at least one magnesium oxide source of step a) is addedfirst followed by the addition of the larger portion to the water inorder to convert the at least one magnesium oxide source of step a) atleast partially into magnesium hydroxide.

The ratio of the at least one magnesium oxide source of step a) to wateris preferably adjusted in such a way that the suspension develops asufficient or suitable viscosity. In one preferred embodiment, aconsiderable excess of water is used, so that the ratio of water to theat least one magnesium oxide source in the suspension is such that theratio (volume of water):(volume of the at least one magnesium oxidesource) is from 40:1 to 3:1, more preferably from 30:1 to 3:1 and mostpreferably from 20:1 to 3:1. For example, if the at least one magnesiumoxide source is selected from caustic calcined magnesite, the ratio ofwater to the caustic calcined magnesite in the suspension may be suchthat the ratio (volume of water):(volume of caustic calcined magnesite)is from 40:1 to 5:1, more preferably from 30:1 to 10:1 and mostpreferably from 20:1 to 15:1. In case where the at least one magnesiumoxide source of step a) is selected from burned dolomite, the ratio ofwater to the burned dolomite in the suspension may be such that theratio (volume of water):(volume of burned dolomite) is from 30:1 to 3:1,more preferably from 20:1 to 3:1 and most preferably from 10:1 to 3:1.In another preferred embodiment, the resulting aqueous suspensioncomprising water and the at least one magnesium oxide source provided instep a) has a solid content of between 1 wt.-% and 20 wt.-% and mostpreferably between 1.5 wt.-% and 17.5 wt.-%, based on the total weightof the suspension.

In one preferred embodiment, said suspension has a lower solid contentif the at least one magnesium oxide source provided in step a) hashigher magnesium oxide content. The term “high content” in the meaningof the present invention refers to an amount of magnesium oxide ormagnesium compound which can be converted into magnesium oxide in thecorresponding magnesium oxide source of at least 70 wt.-%, based on thetotal dry weight of the magnesium oxide source. For example, if the atleast one magnesium oxide source provided in step a) is magnesium oxideand/or a magnesium oxide containing mineral having a high content ofmagnesium oxide and/or a magnesium containing material having a highcontent of a magnesium compound which can be converted into magnesiumoxide, the solid content in the suspension may be in the range between 1wt.-% and 15 wt.-%, more preferably in the range between 1.5 wt.-% and12.5 wt.-% and most preferably in the range between 2 wt.-% and 10 wt.-%based on the total weight of the suspension. Such solid content ispreferably adjusted if the at least one magnesium oxide source providedin step a) is selected from magnesium oxide, periclase, magnesite,magnesium carbonate, magnesium hydroxide and mixtures thereof. Forinstance, if the at least one magnesium oxide source provided in step a)is caustic calcined magnesite obtained by calcining magnesite, the solidcontent in the suspension is preferably in the range between 1 wt.-% and12.5 wt.-%, more preferably in the range between 1.5 wt.-% and 10 wt.-%,even more preferably in the range between 2 wt.-% and 7.5 wt.-% and mostpreferably in the range between 4 wt.-% and 6 wt.-%, for example between4.5 wt.-% and 5.5 wt.-% based on the total weight of the suspension.

In another preferred embodiment, said suspension may have a higher solidcontent if the at least one magnesium oxide source provided in step a)has a lower magnesium oxide content. The term “low content” in themeaning of the present invention refers to an amount of magnesium oxideor magnesium compound which can be converted into magnesium oxide in thecorresponding magnesium oxide of less than 70 wt.-%, based on the totaldry weight of the magnesium oxide source. For example, if the at leastone magnesium oxide source provided in step a) is a magnesium oxidecontaining mineral having a low content of magnesium oxide and/or amagnesium containing material having a low content of a magnesiumcompound which can be converted into magnesium oxide, the solid contentin the suspension may be in the range between 10 wt.-% and 20 wt.-%,more preferably in the range between 10 wt.-% and 17.5 wt.-% and mostpreferably in the range between 12.5 wt.-% and 17.5 wt.-% based on thetotal weight of the suspension. Such solid content is preferablyadjusted if the at least one magnesium oxide source provided in step a)is selected from dolomite, huntite and mixtures thereof. For example, ifthe at least one magnesium oxide source provided in step a) is burneddolomite obtained by calcining dolomite, the solid content in thesuspension is preferably in the range between 10 wt.-% and 20 wt.-%,more preferably in the range between 12.5 wt.-% and 17.5 wt.-% and mostpreferably in the range between 15 wt.-% and 17.5 wt.-%, for examplebetween 16 wt.-% and 17 wt.-% based on the total weight of thesuspension.

Depending on equipment and site conditions, the slaking process ispreferably carried out with water having an elevated temperature inorder to provide small particles of magnesium hydroxide having a highspecific surface and additionally or alternatively in order to obtain asufficient reaction rate. Furthermore, as the incoming water temperaturemay inversely affect the time required for carrying out the slakingprocess, a contacting of the at least one magnesium oxide source withcool water should preferably be avoided. In the case in which cool waterand magnesium oxide come in contact a condition called “drowning” maytake place, resulting in particles of magnesium hydroxide which are verycoarse and not very reactive. Therefore, the temperature of the waterused in the slaking process should preferably be above room temperaturebut below the boiling point of water.

In one preferred embodiment, the temperature of the water added into thereaction vessel for slaking said at least one magnesium oxide sourceprovided in step a) is preferably in the range between 20° C. and 90°C., more preferably in the range between 30° C. and 60° C. and mostpreferably in the range between 35° C. and 55° C., for example 40° C. or50° C.

For example, if the at least one magnesium oxide source provided in stepa) is caustic calcined magnesite obtained by calcining magnesite, saidtemperature is preferably in the range between 20° C. and 45° C., morepreferably in the range between 25° C. and 45° C. and most preferably inthe range between 35° C. and 45° C., for example about 40° C. In thecase where the at least one magnesium oxide source provided in step a)is burned dolomite obtained by calcining dolomite, said temperature ispreferably in the range between 35° C. and 60° C., more preferably inthe range between 40° C. and 55° C. and most preferably in the rangebetween 45° C. and 55° C., for example about 50° C.

During the slaking process the temperature in the reaction vessel variesdue to variation in water temperature, magnesium oxide reactivity, andquality of water and, thus, the temperature of the suspension may beadjusted frequently. Preferably, the temperature is controlledcontinuously. Alternatively, the temperature may be controlledrepeatedly. In another preferred embodiment, the temperature of thesuspension is not adjusted during step c) of the process of the presentinvention is carried out.

In one preferred embodiment, the slaking process of step c) is carriedout by agitation of the suspension. In this respect, agitation can becarried out continuously or discontinuously. However, as the degree ofagitation during the slaking process may have an impact on the obtainedmagnesium hydroxide, the suspension is preferably agitated continuously.In this respect, too little agitation may result in uneven temperaturewithin the suspension resulting in hot and cold spots. Such uneventemperature may result in crystals of large size and reduced surfacearea and agglomeration of particles, while cold spots will result ineither drowning or higher amounts of unhydrated particles of magnesiumoxide.

The slaking process of step c) of the process of the present inventionis preferably conducted to the point where at least a part of themagnesium oxide contained in the at least one magnesium oxide source isconverted to its respective hydroxides, i.e. magnesium hydroxide. Inthis respect, it is to be noted that only a portion of the magnesiumoxide contained in the at least one magnesium oxide source is convertedinto the respective magnesium hydroxide during the slaking process ofstep c). For example, if the at least one magnesium oxide source iscontacted for about 15 min with the slaking water having a temperatureof about 40° C., the amount of magnesium oxide converted into magnesiumhydroxide is in the range between 5 wt.-% and 15 wt.-%, for exampleabout 10 wt.-%, i.e. the magnesium oxide content is in the range between85 wt.-% and 95 wt.-%, for example about 90 wt.-%, based on the totalweight of magnesium oxide and magnesium hydroxide. By contrast, if theat least one magnesium oxide source is contacted for about 30 min withthe slaking water having a temperature of about 40° C., the amount ofmagnesium oxide converted into magnesium hydroxide is in the rangebetween 15 wt.-% and 25 wt.-%, for example about 20 wt.-%, i.e. themagnesium oxide content is in the range between 75 wt.-% and 85 wt.-%,for example about 80 wt.-%, based on the total weight of magnesium oxideand magnesium hydroxide.

The mixture of the at least one magnesium oxide source and magnesiumhydroxide obtained by slaking said magnesium oxide source of step a) ata water temperature of 40° C. and a slaking period of 30 min may have aratio (weight of magnesium oxide):(weight of magnesium hydroxide) whichis preferably from 10:1 to 2:1, more preferably from 8:1 to 3:1 and mostpreferably from 6:1 to 3:1. In this context, it is to be noted thatafter conversion or reaction of the obtained magnesium hydroxide withthe gaseous carbon dioxide and/or carbonate-comprising anions of stepb), further magnesium oxide of the at least one magnesium oxide sourceof step a) in the mixture is converted into magnesium hydroxide whichthen can also be reacted with carbon dioxide and/or carbonate-comprisinganions. In other words, the inventive process can be carried out with amixture of magnesium oxide and magnesium hydroxide (which may beobtained by a partial slaking reaction) since the remaining magnesiumoxide is successively converted into the magnesium hydroxide after themagnesium hydroxide already contained in the starting mixture reactedwith the gaseous carbon dioxide and/or carbonate-comprising anions ofstep b). The at least one magnesium oxide source of step a) may be addedinto the water for carrying out the slaking of step c) of the presentinvention in several portions and/or continuously over the time desiredfor carrying out the process of the present invention and/or until thedesired amount of resulting product is obtained. In said process theamount of water may be adjusted frequently in order to obtain a solidcontent and/or viscosity suitable for carrying out the process of thepresent invention.

It is to be noted that in the case where a magnesium oxide containingmineral having a low content of magnesium oxide and/or a magnesiumcontaining material having a low content of a magnesium compound whichcan be converted into magnesium oxide is used as the magnesium oxidesource provided in step a), said mineral and/or material usuallycomprises a content of magnesium oxide and/or of a magnesium compoundwhich can be converted into magnesium oxide of less than 70 wt.-%, basedon the total dry weight of the magnesium oxide source; i.e. the originalmaterial further comprises other compounds such as carbonates, oxides,hydroxides etc. of alkali metals and/or alkaline earth metals.

For example, if the at least one magnesium oxide source provided in stepa) is dolomite, said mineral is merely made of magnesium carbonate andcalcium carbonate. However, natural products of dolomite possess notonly varying compositions regarding the specific compounds but alsovarying compositions regarding the ratios of said magnesium and calciumcarbonates which occur in a wide range. The ratio of magnesium carbonateto calcium carbonate in naturally occurring dolomite is usually suchthat the ratio (weight of magnesium carbonate):(weight of calciumcarbonate) is from 2:1 to 1:2, more preferably 1.5:1 to 1:1.5 and mostpreferably about 1:1.

Thus, by using dolomite as the magnesium oxide source of step a), thecalcining step carried out prior to the slaking of step c) does not onlyresult in the conversion of magnesium carbonate into magnesium oxide butalso in the conversion of calcium carbonate into calcium oxide in thecorresponding ratios depending on the dolomite used. In the case wheresuch a mixture of obtained magnesium oxide and calcium oxide is slakedby step c), said slaking converts the magnesium oxide at least partiallyinto magnesium hydroxide and in addition thereto, the calcium oxide isalmost completely converted into calcium hydroxide; i.e. by slaking ofburned dolomite in step c) of the process of the present invention, amixture comprising magnesium hydroxide, calcium hydroxide, magnesiumoxide and calcium oxide is obtained. The term “almost completelyconverted” in the meaning of the present invention refers to a reactionin which at least 99 wt.-%, more preferably at least 99.2 wt.-% and mostpreferably at least 99.5 wt.-% of a compound is converted into therespective reaction product. For example, if a mixture of magnesiumoxide and calcium oxide is slaked by step c), said slaking converts atleast 99 wt.-% of calcium oxide, more preferably at least 99.2 wt.-% andmost preferably at least 99.5 wt.-%, based on the total dry weight ofthe calcium oxide, into calcium hydroxide, while the magnesium oxide isonly partially converted into magnesium hydroxide.

The time required for carrying out the slaking process of step c) is thetime required to obtain a sufficient amount of magnesium hydroxide bythe hydration/slaking of the at least one magnesium oxide source forcarrying out step d) of the process of the present invention. This timedepends mainly on the at least one magnesium oxide source provided instep a).

In a preferred embodiment, the at least one magnesium oxide sourceprovided in step a) is slaked for a shorter period of time if the atleast one magnesium oxide source of step a) is in the form of magnesiumoxide and/or a magnesium oxide containing mineral having a high contentof magnesium oxide and/or a magnesium containing material having a highcontent of a magnesium compound which can be converted into magnesiumoxide. Said period of time is preferably in the range between 5 min and30 min, more preferably in the range between 5 min and 20 min and mostpreferably in the range between 10 min and 20 min. Such shorter periodof time is preferably applied for the slaking step if the at least onemagnesium oxide source provided in step a) is selected from magnesiumoxide, periclase, magnesite, magnesium carbonate, magnesium hydroxideand mixtures thereof. For instance, if the at least one magnesium oxidesource provided in step a) is caustic calcined magnesite obtained bycalcining magnesite, said period of time is preferably in the rangebetween 10 min and 30 min, more preferably in the range between 10 minand 25 min and most preferably in the range between 10 min and 20 min,for example about 15 min.

In another preferred embodiment, the at least one magnesium oxide sourceprovided in step a) is slaked for a longer period of time if the atleast one magnesium oxide source provided in step a) is a magnesiumoxide containing mineral having a low content of magnesium oxide and/ora magnesium containing material having a low content of a magnesiumcompound which can be converted into magnesium oxide. Said period oftime is preferably in the range between 5 min and 60 min, morepreferably in the range between 10 min and 45 min and most preferably inthe range between 20 min and 40 min. Such longer period of time ispreferably applied if the at least one magnesium oxide source providedin step a) is selected from dolomite, huntite and mixtures thereof. Forexample, if the at least one magnesium oxide source provided in step a)is burned dolomite obtained by calcining dolomite, said period of timeis preferably in the range between 15 min and 50 min, more preferably inthe range between 15 min and 45 min and most preferably in the rangebetween 25 min and 40 min, for example about 30 min.

After carrying out step c) of the present invention, the obtainedmixture of magnesium hydroxide and magnesium oxide is formed into asuitable suspension for carrying out step d). The overall solid contentof this suspension is preferably in the range between 1 wt.-% and 20wt.-%, more preferably between 1 wt.-% and 15 wt.-% and most preferablybetween 2 wt.-% and 10 wt.-%, based on the total weight of thesuspension obtained in step c). For example, if the magnesium hydroxideis obtained from magnesite as the at least one magnesium oxide source,said overall solid content of the suspension is preferably in the rangebetween 2 wt.-% and 8 wt.-%, more preferably in the range between 3wt.-% and 7 wt.-% and most preferably in the range between 4 wt.-% and 6wt.-%, for example about 5 wt.-%, based on the total weight of thesuspension obtained in step c). If the magnesium hydroxide is obtainedfrom dolomite as the at least one magnesium oxide source, said overallsolid content of the suspension is preferably in the range between 5wt.-% and 10 wt.-%, more preferably in the range between 6 wt.-% and 10wt.-% and most preferably in the range between 7 wt.-% and 9 wt.-%, forexample about 8 wt.-%, based on the total weight of the suspensionobtained in step c).

Additionally or alternatively, the obtained suspension comprisingmagnesium hydroxide and magnesium oxide has preferably a viscosity ofless than 1.000 mPa·s and more preferably of less than 100 mPa·s, asmeasured with a Brookfield DV-II Viscometer at a speed of 100 rpm andequipped with a LV-3 spindle. In the case where the obtained suspensionhas a solid content above or below the desired range and/or theviscosity of said suspension is too high or low the suspension may bediluted with water or up-concentrated by any conventional process knownto the skilled person to obtain a suspension of said desired solidcontent and/or viscosity for the further process steps.

The obtained suspension comprising magnesium hydroxide and magnesiumoxide has preferably a pH of more than 8, more preferably of more than 9and most preferably of more than 10, as measured according to themeasurement method described in the Examples section here below.

Characterization of Step d): Contacting the Obtained Magnesium Hydroxidewith Said Gaseous CO₂ and/or Carbonate-Comprising Anions

According to step d) of the process of the present invention, saidobtained magnesium hydroxide of step c) is contacted with said gaseouscarbon dioxide and/or carbonate-comprising anions of step b) to convertat least a part of the magnesium hydroxide into precipitatednesquehonite.

The magnesium hydroxide is preferably in the form of a suspension, andconsists of water, magnesium hydroxide, unreacted magnesium oxide andimpurities normally associated with magnesium hydroxide suspensions, forexample, silica, calcium oxide, calcium hydroxide and other magnesiumcompounds such as magnesium carbonate etc.

In a preferred embodiment, said suspension has an overall solids contentof at most 20 wt.-%, preferably of at most 15 wt.-%, more preferably ofat most 10 wt.-% and most preferably of between 1 wt.-% and 8.5 wt.-%,based on the total weight of the suspension.

In the case where the suspension comprising magnesium hydroxide andmagnesium oxide is obtained from magnesite as the at least one magnesiumoxide source, the solid content of said suspension is preferably in therange between 2 wt.-% and 8 wt.-%, more preferably in the range between3 wt.-% and 7 wt.-% and most preferably in the range between 4 wt.-% and6 wt.-%, for example in the range between 4.5 wt.-% and 5.5 wt.-%, basedon the total weight of the suspension obtained in step c). If thesuspension comprising magnesium hydroxide and magnesium oxide isobtained from dolomite as the at least one magnesium oxide source ofstep a), the solid content of magnesium hydroxide and magnesium oxide insaid suspension is preferably in the range between 1 wt.-% and 10 wt.-%,more preferably in the range between 2.5 wt.-% and 5 wt.-% and mostpreferably in the range between 3 wt.-% and 5 wt.-%, for example in therange between 3.5 wt.-% and 4.5 wt.-%, based on the total weight of thesuspension obtained in step c).

In one preferred embodiment, said suspension obtained by slaking ofdolomite as the at least one magnesium oxide source provided in step a)further comprises calcium hydroxide. In this case, the solid content ofcalcium hydroxide in the suspension is preferably in the range between 1wt.-% and 10 wt.-%, more preferably in the range between 2.5 wt.-% and 5wt.-% and most preferably in the range between 3 wt.-% and 5 wt.-%, forexample in the range between 3.5 wt.-% and 4.5 wt.-%, based on the totalweight of the suspension obtained in step c). In another preferredembodiment, the solid content of said suspension comprising magnesiumhydroxide, magnesium oxide and calcium hydroxide is preferably in therange between 2 wt.-% and 20 wt.-%, more preferably in the range between2 wt.-% and 10 wt.-%, even more preferably in the range between 5 wt.-%and 10 wt.-% and most preferably in the range between 6 wt.-% and 10wt.-%, for example in the range between 7 wt.-% and 9 wt.-%, based onthe total weight of the suspension obtained in step c).

Additionally or alternatively, the ratio of magnesium oxide andmagnesium hydroxide to calcium hydroxide in the suspension obtained byslaking of burned dolomite as the at least one magnesium oxide source instep c) of the process of the present invention may vary in a widerange. However, in case the suspension of step c) is obtained fromburned dolomite as the at least one magnesium oxide source provided instep a), the ratio of magnesium oxide and magnesium hydroxide to calciumhydroxide in the obtained suspension is preferably such that the ratio(weight of magnesium oxide and magnesium hydroxide):(weight of calciumhydroxide) is from 2:1 to 1:2, more preferably 1.5:1 to 1:1.5 and mostpreferably about 1:1.

In an optional embodiment, the particles of the obtained mixture ofmagnesium hydroxide and magnesium oxide in the suspension may beseparated by their particle size or from impurities prior to processstep d). Preferably, the magnesium hydroxide is separated from particleshaving a particle size larger than 300 μm and more preferably fromparticles having a particle size larger than 200 μm by separationtechnologies known to the skilled person, for example, by vibratingscreens and the like.

Step d) involves contacting the suspension of magnesium hydroxide andmagnesium oxide obtained in step c) with sufficient gaseous CO₂ and/orcarbonate-comprising anions provided in step b) until at least a part ofthe provided magnesium hydroxide is converted to a crystalline magnesiumcarbonate precipitate (precipitated magnesium carbonate). In thiscontext, it is to be noted, that the formation of said crystallinemagnesium carbonate precipitate may lead to a conversion of remainingmagnesium oxide in the suspension to magnesium hydroxide, which may befurther converted to said crystalline magnesium carbonate precipitate bycontacting the obtained magnesium hydroxide with sufficient gaseous CO₂and/or carbonate-comprising anions provided in step b). The carbonationis continued until substantially all of the magnesium is precipitated,so that the suspension is composed almost entirely of a crystallinemagnesium carbonate precipitate. Said crystalline magnesium carbonateprecipitate is characterized as being nesquehonite having the formulaMg(HCO₃)(OH).2H₂O, which may also be described as being MgCO₃.3H₂O. Theprecipitated nesquehonite crystals obtained are of a prismatic elongatedtype being typical for nesquehonite.

For contacting the suspension comprising magnesium hydroxide andmagnesium oxide obtained in step c) with said gaseous CO₂ of step b),the gas is preferably bubbled through the suspension. By bubbling thegaseous CO₂ through the suspension, a sufficient mixing may be achievedby the flow of the gas in the suspension, so that an additionalagitation is not required. Additionally or alternatively, the suspensioncomprising magnesium hydroxide and magnesium oxide is agitated, whichmay provide a more thorough mixing and thus a shorter period of time forcompleting the conversion of magnesium hydroxide into magnesiumcarbonate, namely precipitated nesquehonite. In a preferred embodiment,the suspension comprising magnesium hydroxide and magnesium oxide isadditionally agitated to ensure a thorough mixing of the particles inorder to provide a sufficient amount of unreacted magnesium hydroxideparticles for contacting the particles with said CO₂. Such agitation canbe carried out continuously or discontinuously as long as the mixingprovides a sufficient conversion of magnesium hydroxide into magnesiumcarbonate. In one preferred embodiment, the suspension is preferablyagitated continuously.

In one preferred embodiment, said gaseous CO₂ is preferably added to thesuspension comprising magnesium hydroxide and magnesium oxide bybubbling the carbon dioxide through the suspension in a constant rate.Said rate is preferably in the range between 0.1 and 10 kg CO₂/h per kgmagnesium oxide, more preferably in the range between 0.2 and 5 kg CO₂/hper kg magnesium oxide and most preferably in the range between 0.5 and2 kg CO₂/h per kg magnesium oxide.

Preferably, the ratio of suspension comprising magnesium hydroxide andmagnesium oxide to gaseous CO₂ in the aqueous suspension is, in terms ofvolume, such that the ratio (volume of suspension):(volume of gaseousCO₂) is from 1:0.5 to 1:10 and more preferably 1:0.5 to 1:5. In onepreferred embodiment, the ratio of magnesium hydroxide in the suspensionto gaseous CO₂ is, in terms of volume, such that the ratio (volume ofmagnesium hydroxide):(volume of gaseous CO₂) is from 1:2 to 1:100 andmore preferably 1:5 to 1:50.

In one preferred embodiment, the carbonation; i.e. the conversion ofmagnesium hydroxide is monitored by the change of the pH value and/orthe electrical conductivity and/or temperature and/or CO₂ content in theoffgas in order to control the progress or completion of the reaction.

For instance, if said crystalline magnesium carbonate precipitate isobtained from caustic calcined magnesite as the at least one magnesiumoxide source provided in step a), the pH of the suspension comprisingmagnesium oxide and magnesium hydroxide prior to step d) of the processof the present invention is preferably in the range between pH 10 and12, approximately about pH 11. In one preferred embodiment, the pH ofsaid suspension decreases during contacting the obtained magnesiumhydroxide of step c) with said gaseous CO₂ of step b) such that theobtained suspension after carrying out process step d) has a pH in therange between 7 and 8, approximately between pH 7.5 and 8.

By contrast, if said crystalline magnesium carbonate precipitate isobtained from burned dolomite as the at least one magnesium oxide sourceprovided in step a), the pH of the suspension comprising magnesium oxideand magnesium hydroxide and calcium hydroxide prior to step d) of theprocess of the present invention is preferably above pH 11,approximately about pH 12. In one preferred embodiment, the pH of saidsuspension decreases during contacting the obtained magnesium hydroxideof step c) with said gaseous CO₂ of step b) such that the obtainedsuspension after carrying out process step d) has a pH in the rangebetween 7 and 8, approximately between pH 7.5 and 8.

The temperature provided at the beginning of step d) of the presentinvention is decisive for controlling the formation of the resultingprecipitated nesquehonite or its properties even though the temperatureemployed may vary within a specific range. For example, the startingtemperature of the carbonation step provided in step d) may be adjustedto a temperature in the range between 5° C. and 35° C. and mostpreferably in the range between 10° C. and 30° C.

The temperature in the suspension may preferably be controlled andmaintained at said starting temperature while step d) is carried out. Inthis respect, it is to be noted that the term “the temperature ismaintained” during said process step in the meaning of the presentinvention relates to a temperature which does preferably not exceed thestarting temperature by more than 5° C.; i.e. if the startingtemperature is for example adjusted to a temperature of 25° C., thetemperature during the process step may not exceed 30° C. For example,if the at least one magnesium oxide source provided in step a) iscaustic calcined magnesite obtained by calcining magnesite, saidstarting temperature at the beginning of process step d) is preferablyin the range between 20° C. and 28° C. and most preferably in the rangebetween 24° C. and 26° C. During step d) is carried out, the temperatureis preferably controlled and maintained between 20° C. and 25° C. Asanother example, if the at least one magnesium oxide source provided instep a) is burned dolomite obtained by calcining dolomite, said startingtemperature at the beginning of process step d) is preferably in therange between 20° C. and 28° C. and most preferably in the range between24° C. and 26° C. During step d) is carried out, the temperature ispreferably controlled and maintained between 20° C. and 30° C.

In another preferred embodiment, the starting temperature of processstep d) is allowed to rise while step d) is carried out. However, due tothe exothermic reaction the temperature of the reaction mixture may riseto temperatures of 50° C. and more. The maximum temperature in thisembodiment of the process is preferably not more than 50° C. and mostpreferably the maximum temperature reached during step d) is not morethan about 45° C. If the temperature is allowed to rise during step d)is carried out, the starting temperature adjusted is preferably in therange of between 5° C. and 15° C. Such setting is preferably applied ifthe at least one magnesium oxide source provided in step a) is selectedfrom dolomite, huntite and mixtures thereof. For example, if the atleast one magnesium oxide source provided in step a) is burned dolomiteobtained by calcining dolomite, said starting temperature during step d)of the process of the present invention is preferably in the rangebetween 7° C. and 15° C., more preferably in the range between 10° C.and 15° C. and most preferably in the range between 11° C. and 13° C.During step d) is carried out, the temperature is allowed to rise suchthat the temperature rises to a maximum temperature of at most 50° C.,preferably between 40° C. and 45° C.

In case the suspension comprising magnesium hydroxide and magnesiumoxide obtained in step c) is contacted with said carbonate-comprisinganions of step b), the carbonate-comprising anions are preferably addedto said suspension in any appropriate solid form, e.g. in the form ofgranules or a powder or in the form of a suspension or solution. In onepreferred embodiment, the suspension comprising magnesium hydroxide andmagnesium oxide is agitated during the addition of thecarbonate-comprising anions, which may provide a more thorough mixingand thus a shorter period of time for completing the conversion ofmagnesium hydroxide into magnesium carbonate, namely precipitatednesquehonite. Such agitation can be carried out continuously ordiscontinuously as long as the mixing provides a sufficient conversionof magnesium hydroxide into magnesium carbonate. In one preferredembodiment, the suspension is preferably agitated continuously.

Preferably, the concentration of the carbonate-comprising anions in thesuspension comprising magnesium oxide and magnesium hydroxide is suchthat the weight ratio of said suspension:carbonate-comprising anions isfrom 300:1 to 10:1, more preferably 250:1 to 25:1, and even morepreferably 200:1 to 50:1.

By carrying out step d) of the process of the present invention aprecipitated intermediate product is obtained by contacting the obtainedsuspension of magnesium hydroxide of step c) with the gaseous CO₂ and/orcarbonate-comprising anions of step b). Said precipitated intermediateproduct is characterized as being nesquehonite having the formulaMg(HCO₃)(OH).2H₂O, which may also be described as being MgCO₃.3H₂O. Theprecipitated nesquehonite crystals obtained are of a prismatic elongatedtype being typical for nesquehonite.

Accordingly, the time required for carrying out the carbonation of stepd) is the time required to almost complete the conversion of themagnesium hydroxide obtained in step c) into precipitated nesquehonite.Such almost complete conversion of magnesium hydroxide into precipitatednesquehonite is preferably obtained within 4 hours, more preferablywithin 3 hours, even more preferably within 2 hours and most preferablywithin 90 min, calculated from the start of contacting the at leastpartially obtained magnesium hydroxide of step c) with said gaseous CO₂and/or carbonate-comprising anions.

The precipitated nesquehonite obtained is preferably in the form of anaqueous suspension. It has been found that a suspension of precipitatednesquehonite having a solid content in the suspension of up to 50 wt.-%,preferably between 1 and 50 wt.-%, more preferably between 1 and 25wt.-% and most preferably between 5 and 15 wt.-%, based on the totalweight of the suspension, is preferred. In the case where the obtainedsuspension has a solid content above or below the desired range thesuspension may be diluted with water or up-concentrated by anyconventional process known to the skilled person to obtain a suspensionof said desired solid content for the further process step.

In an optional embodiment, the particles of the obtained precipitatednesquehonite in the suspension may be separated by their particle sizeor from impurities prior to the heat-ageing step. In one preferredembodiment, the precipitated nesquehonite is separated from particleshaving an average particle size d₅₀ value of more than 200 μm, morepreferably from particles having an average particle size d₅₀ value ofmore than 150 μm and most preferably from particles having an averageparticle size d₅₀ value of more than 100 μm, as measured by screeningand determining mass fractions of selected size ranges.

In this context, it is to be noted that the average particle size d₅₀value of the obtained precipitated nesquehonite may vary in a broadrange but, in general, the particles of the obtained precipitatednesquehonite have an average particle size d₅₀ value of less than 50 μm,more preferably of less than 35 μm, even more preferably of less than 20μm and most preferably of less than 15 μm, as determined by laserdiffraction using the instrument CILAS 920 particle-size-analyzer ofCILAS, Orleans, France.

For example, if the at least one magnesium oxide source is derived frommagnesite in the form of caustic calcined magnesite, the particles ofthe obtained precipitated nesquehonite in the suspension preferably havean average particle size d₅₀ value of less than 30 μm, more preferablyof less than 25 μm, even more preferably of less than 20 μm and mostpreferably between 10 μm and 15 μm, as determined by laser diffractionusing the instrument CILAS 920 particle-size-analyzer of CILAS, Orleans,France.

By contrast, if the precipitated nesquehonite of step d) is obtainedfrom burned dolomite as the at least one magnesium oxide source providedin step a), the suspension obtained in step d) may further compriseprecipitated calcium carbonate (PCC). In this case, the mixture of theobtained particles of precipitated nesquehonite and precipitated calciumcarbonate in the suspension preferably have an overall average particlesize d₅₀ value of less than 20 μm, more preferably of less than 15 μm,even more preferably of less than 10 μm and most preferably of less than7.5 μm, for example less than 5 μm, as determined by laser diffractionby using the instrument CILAS 920 particle-size-analyzer of CILAS,Orleans, France.

In an especially preferred embodiment, the particles of the obtainedprecipitated nesquehonite in the suspension are ground prior to theheat-ageing step in order to provide particles having a reduced particlesize and/or in order to provide particles of about equal diameter. Thegrinding step can be carried out with any conventional grinding devicesuch as a grinding mill known to the skilled person. In one preferredembodiment, the precipitated nesquehonite particles in the aqueoussuspension are wet-ground in a vertical bead mill. Preferably, saidwet-grinding is carried out at a specific energy input during grindingin the range between 10 kWh/dry ton to 500 kWh/dry ton, more preferablyat a specific energy input during grinding in the range between 20kWh/dry ton to 300 kWh/dry ton and most preferably at a specific energyinput during grinding in the range between 50 kWh/dry ton to 200 kWh/dryton, for example at a specific energy input during grinding of about 100kWh/dry ton.

The intermediate grinding step is especially advantageous if themagnesium oxide source is derived from magnesite in the form of causticcalcined magnesite. Accordingly, if the precipitated nesquehoniteparticles are ground prior to the heat-ageing of step e) of the processof the present invention, the nesquehonite particles obtained fromcaustic calcined magnesite as the at least one magnesium oxide source ofstep a) preferably have an average particle size d₅₀ value of less than25 μm, more preferably of less than 20 μm, even more preferably of lessthan 15 μm and most preferably of less than 10 μm, as determined bylaser diffraction using the instrument CILAS 920 particle-size-analyzerof CILAS, Orleans, France.

However, in case where the mixture of particles of precipitatednesquehonite and precipitated calcium carbonate obtained from burneddolomite as the at least one magnesium oxide source of step a) is groundprior to the heat-ageing step, the particles in the obtained mixture ofprecipitated nesquehonite and precipitated calcium carbonate preferablyhave an overall average particle size d₅₀ value of less than 10 μm, morepreferably of less than 7.5 μm, even more preferably of less than 5 μmand most preferably of less than 3 μm, for example less than 2.5 μm, asdetermined by laser diffraction by using the instrument CILAS 920particle-size-analyzer of CILAS, Orleans, France.

It is further to be noted that if the precipitated nesquehonite of stepd) is obtained from burned dolomite as the at least one magnesium oxidesource provided in step a), the ratio of precipitated nesquehonite andprecipitated calcium carbonate in the suspension obtained in step d) ofthe process of the present invention may vary in a wide range.Preferably, the ratio of precipitated nesquehonite to precipitatedcalcium carbonate in the obtained suspension is preferably such that theratio (weight of precipitated nesquehonite):(weight of precipitatedcalcium carbonate) is from 3:1 to 1:3, more preferably 2:1 to 1:2 andmost preferably from 1.5:1 to 1:1.5.

Additionally or alternatively, a polysaccharide may be added into thesuspension before step d) of the process of the present invention iscarried out; i.e. the suspension containing the at least one magnesiumoxide source contains said polysaccharide during the carbonation of stepd). The polysaccharide is preferably selected from the group consistingof sorbitol, mannitol, sucrose and mixtures thereof. In one preferredembodiment, the polysaccharide is sorbitol.

The polysaccharide is preferably added into the suspension in a quantityso that it is contained in the resulting suspension in a concentrationbetween 0.001 wt.-% and 5 wt.-%, more preferably between 0.01 wt.-% and0.1 wt.-% and most preferably between 0.05 wt.-% and 0.75 wt.-%, basedon the total weight of the suspension.

The polysaccharide can be added to the suspension in any appropriatesolid form, e.g. in the form of granules or a powder. Alternatively, thepolysaccharide can be added to the suspension in the form of asuspension or solution.

Characterization of Step e): Treating the Obtained PrecipitatedNesquehonite in a Heat-Ageing Step

According to step e) of the process of the present invention, saidobtained precipitated nesquehonite of step d) is treated by aheat-ageing in order to obtain the hydromagnesite.

The term “heat-ageing” in the meaning of the present invention relatesto a thermal process in which crystals, such as of nesquehonite, havinginitially a higher internal energy state, undergo a phase transformationby dissolving and redepositing into crystals having a lower internalenergy state. The process results in a final crystal productcharacterized by greater perfection of its crystal lattice structure, anarrower particle size distribution, greater degree of particlediscreteness and lower surface energy.

The heat-ageing of the precipitated nesquehonite obtained in step d) maybe carried out at temperatures of above 90° C. and most preferably at atemperature in the range between 90° C. and 150° C., wherein saidtemperature range reflects the period required for converting theobtained precipitated nesquehonite into hydromagnesite; i.e. the higherthe temperature at which the heat-ageing is carried out the lower thetime required for achieving an almost complete conversion ofprecipitated nesquehonite into hydromagnesite or the lower thetemperature at which the heat-ageing is carried out the higher the timerequired for achieving an almost complete conversion of precipitatednesquehonite into hydromagnesite. Preferably, the precipitatednesquehonite is maintained at the heat-ageing temperature for asufficient time to cause the morphology of the nesquehonite to rearrangeto the final form of the final product hydromagnesite. In this respect,the period required for achieving an almost complete conversion intohydromagnesite starting from the precipitated nesquehonite may varybetween 10 min and several hours depending on the temperature appliedduring said heat-ageing step.

The period of time the precipitated nesquehonite should be maintained atthe heat-ageing temperature in order to recrystallize to the newmorphology having decreased particle size is determined by both theinitial morphology of the precipitated nesquehonite and the nature andextent of any impurities present in the magnesium carbonate. Forexample, where the precipitated nesquehonite material has a smallinitial average particle size, the period of time for said ageing stepat about 130° C. is as short as, for example, about 30 minutes.

In order to arrive at specifically small particles of hydromagnesite theheat-ageing process for converting the precipitated nesquehonite intohydromagnesite is preferably carried out at temperatures is in the rangebetween 90° C. and 150° C., preferably in the range between 110° C. and140° C., more preferably in the range between 120° C. to 135° C. andmost preferably at a temperature of about 130° C.

For instance, if the heat-ageing temperature of the precipitatednesquehonite is adjusted to a temperature of about 130° C., saidtemperature is preferably maintained for a period of time of more than10 min and more preferably for a period of time of between 20 min and 60min. In one preferred embodiment, the heat-ageing temperature ismaintained for a period of time of between 20 min and 40 min, morepreferably for a period of time of between 25 and 35 min and mostpreferably for about 30 min. The heat-ageing reaction can be monitoredby measuring the surface area and/or conductivity of the hydromagnesiteat specific intervals.

In an optional embodiment, a bleaching agent is added into thesuspension of nesquehonite obtained in step d); i.e. said bleachingagent is added prior the heat-ageing of step e) is carried out.Additionally or alternatively, the bleaching agent may be added duringthe carbonation step, i.e. during the step in which the obtainedmagnesium hydroxide of step c) is contacted with the gaseous CO₂ of stepb).

In one preferred embodiment, the bleaching agent is sodium dithionite.In a further preferred embodiment, the bleaching agent is formamidinesulfinic acid. Alternatively or additionally other suitable bleachingagents may be used.

The bleaching agent is preferably added into the correspondingsuspension in a quantity so that it is contained in the resultingsuspension of magnesium hydroxide and precipitated nesquehonite,respectively, in a concentration between 0.001 wt.-% and 10 wt.-%, morepreferably between 0.01 wt.-% and 1 wt.-% and most preferably between0.05 wt.-% and 0.5 wt.-%, based on the total weight of the suspension.

The bleaching agent can be added to the corresponding suspension in anyappropriate solid form, e.g. in the form of granules or a powder.Alternatively, the bleaching agent can be added to the correspondingsuspension in the form of a suspension or solution.

By using the process of the present invention, it is possible to providehydromagnesite particles having a specifically decreased particle size.Preferably the obtained hydromagnesite particles have an averageparticle size d₅₀ value in the range of less than 20 μm, preferably ofless than 15 μm, more preferably of less than 10 μm and most preferablyof less than 5 μm, as determined by laser diffraction using theinstrument CILAS 920 particle-size-analyzer of CILAS, Orleans, France.

In one preferred embodiments, it is possible to obtain a hydromagnesitesuspension having a high content of hydromagnesite if saidhydromagnesite is obtained from a magnesium oxide source in the form ofmagnesium oxide and/or a magnesium oxide containing mineral having ahigh content of magnesium oxide and/or a magnesium containing materialhaving a high content of a magnesium compound which can be convertedinto magnesium oxide. Said content of hydromagnesite is preferably above85 wt.-%, more preferably above 90 wt.-% and most preferably above 95wt.-%, based on the total weight of the solid content in the suspension.For example, if magnesite is used as the at least one magnesium oxidesource provided in step a), the content of hydromagnesite is preferablyabove 90 wt.-%, more preferably above 93.5 wt.-% and most preferablyabove 97 wt.-%, based on the total weight of the solid content in thesuspension.

Furthermore, if said hydromagnesite is obtained from a magnesium oxidesource in the form of magnesium oxide and/or a magnesium oxidecontaining mineral having a high content of magnesium oxide and/or amagnesium containing material having a high content of a magnesiumcompound which can be converted into magnesium oxide, said processprovides hydromagnesite particles having a specifically decreasedparticle size. Preferably the obtained hydromagnesite particles have anaverage particle size d₅₀ value in the range of less than 20 μm,preferably in the range of 0.1 μm to 15 μm, more preferably in the rangeof 0.5 μm to 10 μm, and most preferably in the range of 1 μm to 5 μm,for example in the range between 4.75 μm to 5 μm, as determined by laserdiffraction using the instrument CILAS 920 particle-size-analyzer ofCILAS, Orleans, France. Additionally, by using the process of thepresent invention, the particles obtained are preferably of a platy-likemorphology. Particles having the foregoing characteristics arepreferably obtained if the at least one magnesium oxide source of stepa) in the form of magnesium oxide and/or a magnesium oxide containingmineral having a high content of magnesium oxide and/or a magnesiumcontaining material having a high content of a magnesium compound whichcan be converted into magnesium oxide is selected from magnesium oxide,periclase, magnesite, magnesium carbonate, magnesium hydroxide andmixtures thereof.

In a preferred embodiment, particles having the foregoingcharacteristics are preferably obtained if the at least one magnesiumoxide source of step a) is magnesite and most preferably magnesite inthe form of caustic calcined magnesite.

If said hydromagnesite is obtained from a magnesium oxide source in theform of a magnesium oxide containing mineral having a low content ofmagnesium oxide and/or a magnesium containing material having a lowcontent of a magnesium compound which can be converted into magnesiumoxide, the content of the hydromagnesite in the resulting suspension ispreferably in the range between 20 wt.-% and 70 wt.-%, based on thetotal weight of the solid content in the suspension. In the case wherethe hydromagnesite is obtained from dolomite, the content of thehydromagnesite in the resulting suspension may be for example in therange between 30 wt.-% and 60 wt.-% and more preferably in the rangebetween 35 wt.-% and 50 wt.-%, based on the total weight of the solidcontent in the suspension.

Additionally, if the at least one magnesium oxide source provided instep a) is dolomite, it is to be noted that by using the process of thepresent invention, hydromagnesite is obtained in admixture withprecipitated calcium carbonate, wherein the obtained compositioncomprises hydromagnesite having a platy-like morphology and precipitatedcalcium carbonate having a colloidal morphology.

In the case where hydromagnesite is obtained in admixture withprecipitated calcium carbonate, the content of the precipitated calciumcarbonate in the resulting suspension may be for example in the rangebetween 40 wt.-% and 70 wt.-% and more preferably in the range between50 wt.-% and 65 wt.-%, based on the total weight of the solid content inthe suspension.

Furthermore, by using the inventive process, it is possible to obtainhydromagnesite particles in admixture with other particles having adecreased particle size if said hydromagnesite is obtained from amagnesium oxide source in the form of a magnesium oxide containingmineral having a low content of magnesium oxide and/or a magnesiumcontaining material having a low content of a magnesium compound whichcan be converted into magnesium oxide. Said process preferably provideshydromagnesite particles in admixture with other particles having anaverage particle size d₅₀ value in the range of up to 15 μm, preferablyin the range of 0.1 μm to 10 μm, more preferably in the range of 0.5 μmto 5 μm and most preferably in the range of 1 μm to 4 μm, for example,in the range between 3.25 μm and 3.5 μm, as determined by laserdiffraction using the instrument CILAS 920 particle-size-analyzer ofCILAS, Orleans, France. Particles having said characteristics arepreferably obtained if the at least one magnesium oxide source of stepa) is selected from dolomite, huntite and mixtures thereof.

In a preferred embodiment, particles having the foregoingcharacteristics are preferably obtained if the at least one magnesiumoxide source of step a) is dolomite and most preferably dolomite in theform of burned dolomite and/or half-burned dolomite.

In a preferred embodiment, the obtained hydromagnesite of the presentinvention is preferably in the form of a suspension, wherein the solidcontent can be adjusted to any solid content suitable for application inthe paper, paint, rubber and plastics industries. In this respect, it isto be noted that the obtained hydromagnesite can be used directlywithout carrying out further treatment steps.

In one preferred embodiment, hydromagnesite in the form of a suspensionhas a solid content of up to 30 wt.-%, preferably between 1 wt.-% and 20wt.-%, more preferably between 5 wt.-% and 15 wt.-% and most preferablybetween 7 wt.-% and 11 wt.-%, based on the total weight of thesuspension.

In another preferred embodiment, said suspension preferably has a pHvalue in the range of 6 to 11, preferably a pH value of 7 to 10.5 andmore preferably a pH value of 8.5 to 10.5. The viscosity is preferablyless than 2.500 mPa·s, more preferably less than 2.000 mPa·s and mostpreferably less than 1.750 mPa·s, as measured with a Brookfield DV-IIViscometer at a speed of 100 rpm and equipped with a LV-3 spindle.

In a preferred embodiment, the aqueous phase of the obtainedhydromagnesite suspension may be replaced with deionised water. In anoptional embodiment, the obtained hydromagnesite suspension may beconcentrated, optionally up to the point of obtaining a dryhydromagnesite product. If the aqueous suspension described above isdried, the obtained solids (i.e. dry or containing as little water thatit is not in a fluid form) of hydromagnesite may be in the form ofgranules or a powder. In the case of a dry product, this product mayadditionally be treated with fatty acids during and/or before and/orafter drying. Said fatty acids are preferably selected from stearicacid, palmitic acid, behenic acid, montanic acid, capric acid, lauricacid, myristic acid, isostearic acid and cerotic acid.

The hydromagnesite obtained from magnesium oxide sources by the processof the present invention are of distinct platy-like morphology incombination with specifically decreased particle size and, thus, allowsfor easy and economic applications in the paper, paint, rubber andplastics industries. The particles of the hydromagnesite obtainedaccording to the present invention, have a particle size distribution,wherein the obtained particles have an average particle size d₅₀ valuein the range of less than 20 μm, preferably of less than 15 μm, morepreferably of less than 10 μm and most preferably of less than 5 μm. Ina preferred embodiment, the process of the present invention may providehydromagnesite particles having an average particle size d₅₀ value ofless than 20 μm, preferably in the range of 0.1 μm to 15 μm, morepreferably in the range of 0.5 μm to 10 μm, and most preferably in therange of 1 μm to 5 μm, for example in the range between 4.75 μm to 5 μm.In another preferred embodiment, the process of the present inventionmay provide hydromagnesite particles having an average particle size d₅₀value in the range of up to 15 μm, preferably in the range of 0.1 μm to10 μm, more preferably in the range of 0.5 μm to 5 μm and mostpreferably in the range of 1 μm to 4 μm, for example, in the rangebetween 3.25 μm and 3.5 μm, as determined by laser diffraction using theinstrument CILAS 920 particle-size-analyzer of CILAS, Orleans, France.

In one preferred embodiment, the obtained hydromagnesite provides orshows an absolute density of above 2.25 g/cm³, more preferably thedensity is between 2.26 g/cm³ and 2.40 g/cm³, even more preferably thedensity is between 2.26 g/cm³ and 2.35 g/cm³ and most preferably thedensity is between 2.26 g/cm³ and 2.32 g/cm³. For example, if thehydromagnesite is obtained from caustic calcined magnesite as the atleast one magnesium oxide source provided in step a), the density of thehydromagnesite may be about 2.29 g/cm³.

In another preferred embodiment, the obtained hydromagnesite provides aspecific BET surface area of 10 m²/g to 150 m²/g, more preferably 10m²/g to 100 m²/g and most preferably 20 m²/g to 70 m²/g, as measuredusing nitrogen and the BET method according to ISO 9277.

For example, if the hydromagnesite is obtained from caustic calcinedmagnesite as the at least one magnesium oxide source provided in step a)said hydromagnesite preferably features a specific BET surface area of10 m²/g to 70 m²/g, more preferably of 20 m²/g to 50 m²/g and mostpreferably of 25 m²/g to 40 m²/g, for example of 30 m²/g to 35 m²/g, asmeasured using nitrogen and the BET method according to ISO 9277. In thecase where the hydromagnesite is obtained from burned dolomite as the atleast one magnesium oxide source provided in step a), i.e. the resultingsuspension of step e) also comprises precipitated calcium carbonate,said composition preferably features a specific BET surface area of 40m²/g to 100 m²/g, more preferably of 45 m²/g to 80 m²/g and mostpreferably of 50 m²/g to 70 m²/g, for example of 55 m²/g to 65 m²/g, asmeasured using nitrogen and the BET method according to ISO 9277.

In another preferred embodiment, the obtained hydromagnesite has aspecific BET surface area within the range of 20 to 50 m²/g and theparticles have an average particle size d₅₀ value of less than 20 μm,preferably in the range of 0.1 μm to 15 μm, more preferably in the rangeof 0.5 μm to 10 μm, and most preferably in the range of 1 μm to 5 μm,for example, in the range between 4.75 μm to 5 μm, as determined bylaser diffraction using the instrument CILAS 920 particle-size-analyzerof CILAS, Orleans, France.

Even more preferably the specific BET surface area is within the rangeof 45 to 80 m²/g and the particles have a average particle size d₅₀value in the range of up to 15 μm, preferably in the range of 0.1 μm to10 μm, more preferably in the range of 0.5 μm to 5 μm and mostpreferably in the range of 1 μm to 4 μm, for example, in the rangebetween 3.25 μm and 3.5 μm, as determined by laser diffraction using theinstrument CILAS 920 particle-size-analyzer of CILAS, Orleans, France.

Furthermore, it is preferred that the obtained hydromagnesite has adegree of whiteness R457, measured in accordance with the ISO 2469Standard, of at least 80%, more preferably of at least 85%, even morepreferably of between 85% and 99% and most preferably of between 85% and99%. In another preferred embodiment, the obtained hydromagnesite has adegree of whiteness R457, measured in accordance with the ISO 2469Standard, of at least 89% and more preferably between 89% and 99%. In afurther preferred embodiment, the obtained hydromagnesite has a degreeof whiteness R457, measured in accordance with the ISO 2469 Standard, ofat least 93%. Additionally or alternatively, the hydromagnesite obtainedby the process of the process of the present invention has a YellownessIndex according to DIN 6167 of less than 5, more preferably of less than4 and most preferably of less than 3.

If the hydromagnesite is provided in the form of a suspension, saidhydromagnesite is optionally dispersed. Conventional dispersants knownto the skilled person can be used. The dispersant can be anionic orcationic. A preferred dispersant is one based on polyacrylic acid. Suchdispersants are preferably dosed so as to account for about 0.3 wt.-% toabout 3 wt.-%, based on the total weight of said hydromagnesite.

The hydromagnesite thus obtained may be used in paper, tissue paper,plastics or paints. In particular, said hydromagnesite can be used asmineral filler and/or for coating of paper and in particular as mineralfiller in paper wrappers for smoking articles. In particular, coatingcompositions and/or mineral filler compositions according to theinvention are characterized in that they contain hydromagnesite obtainedby the process of the present invention and in that they provideimproved optical properties in comparison to compositions comprisinghydromagnesite of the prior art. Papers and in particular paper wrapperfor smoking articles manufactured and/or coated are characterized inthat they contain said hydromagnesite obtained by the process of thepresent invention. As another advantage, the hydromagnesite obtained bythe process of the present invention can be used directly in a papermaking application without the removal of, for example, impurities suchas other salts or colored compounds. Furthermore, the obtainedhydromagnesite may be used as flame-retardants having non-conductiveproperties and further functions as electrical insulators. Suchflame-retardants may be incorporated in electric and electronic parts,constructional materials, waste pipes, gutter, automobile parts,cabinets for televisions, computers and similar equipments, profiles andfittings such as fittings for cables, electric switches, sealants,plasters and paints. Flame-retardants comprising hydromagnesite may thuspreferably used in building industry, ships, aircrafts, trains andvehicles.

The following examples may additionally illustrate the invention, butare not meant to restrict the invention to the exemplified embodiments.The examples below show the good optical properties such as opacity ofthe calcium carbonate suspensions according to the present invention:

EXAMPLES

Measurement Methods

The following measurement methods are used to evaluate the parametersgiven in the examples and claims.

Brookfield Viscosity

The Brookfield-viscosity of a slurry was determined with a BrookfieldViscometer type RVT equipped with a LV-3 spindle at a speed of 100 rpmand room temperature (20±3° C.).

BET Specific Surface Area of a Material

The BET specific surface area is measured via the BET method accordingto ISO 4652 using nitrogen, following conditioning of the sample byheating at 250° C. for a period of 30 minutes. Prior to suchmeasurements, the sample is filtered, rinsed and dried at 110° C. in anoven for at least 12 hours.

Particle Size Distribution (Mass % Particles with a Size <X) and AverageParticle Size (d₅₀) of a Particulate Material

The average particle size and the average particle size massdistribution of a particulate material are determined via laserdiffraction, i.e. the light from a laser passes though a suspension andthe particle size distribution is calculated from the resultingdiffraction pattern. The measurement is made with a CILAS 920particle-size-analyzer of CILAS, Orleans, France.

The method is well known to the skilled person and is commonly used todetermine the particle size distribution of particulate materials. Themeasurement is carried out by diluting the corresponding suspension(deionised water; solution of 0.1 wt.-% of sodium pyrophosphate). Thesamples were dispersed using a high speed stirrer and ultrasonic.

pH of an Aqueous Suspension

The pH of the aqueous suspension is measured using a standard pH-meterat approximately 22° C.

Density of Solid Particles

The density of the product is measured using the standard densityanalyzer Micromeritics AccuPyc® commercialized by Micromeritics.

Solids Content of an Aqueous Suspension

The suspension solids content (also known as “dry weight”) is determinedusing a Moisture Analyser HR73 commercialized by Mettler-Toledo with thefollowing settings: temperature of 120° C., automatic switch off 3,standard drying, 5-20 g of suspension.

Comparative Example

The following comparative example illustrates the preparation ofhydromagnesite by a process of the prior art. Said process is carriedout by slaking caustic calcined magnesite and contacting the obtainedmagnesium hydroxide with gaseous CO₂ to convert the obtained magnesiumhydroxide into hydromagnesite, wherein the carbonation is carried out ata starting temperature of about 60° C.

90 kg of caustic calcined magnesite (Van Mannekus M95) were slaked byadding said magnesite to 1 700 liters of 40° C. tap water in a stirredreactor. The magnesite was slaked for 15 min under continuous stirringand the resulting suspension was adjusted to about 5% solids content viadilution with water. The carbonation was conducted in a 1 800 literbaffled cylindrical stainless steel reactor equipped with a gasingagitator, a stainless steel carbonation tube to direct a carbondioxide/air gas stream to the impeller and probes for monitoring the pHand conductivity of the suspension. The suspension obtained in theslaking step was adjusted to a temperature of 60° C. and added to thecarbonating reactor. A gas of 26% by volume of CO₂ in air was thenbubbled upwards through the suspension at a rate of 200 m³/h under aslurry agitation of 240 rpm. During the carbonation, the temperature ofthe reaction mixture was not controlled. After 85 minutes (calculatedfrom start of introduction of said gas) the introduction of gas wasstopped. The product was recovered as an aqueous suspension.Characteristics and physical properties are given in column A of table1.

Example 1

The following illustrative example of the invention involves thepreparation of hydromagnesite by slaking caustic calcined magnesite andcontacting the obtained magnesium hydroxide with gaseous CO₂ to convertthe obtained magnesium hydroxide into hydromagnesite, wherein thecarbonation is carried out with a starting temperature of about 20° C.to 25° C. and a subsequent heat-ageing step.

90 kg of caustic calcined magnesite (Van Mannekus M95) were slaked byadding said magnesite to 1 700 liters of 40° C. tap water in a stirredreactor. The magnesite was slaked for 15 min under continuous stirringand the resulting suspension was adjusted to about 5% solids content viadilution with water. The carbonation was conducted in a 1800 literbaffled cylindrical stainless steel reactor equipped with a gasingagitator, a stainless steel carbonation tube to direct a carbondioxide/air gas stream to the impeller and probes for monitoring the pHand conductivity of the suspension. The suspension obtained in theslaking step was adjusted to a temperature of about 25° C. and added tothe carbonating reactor. A gas of 26% by volume of CO2 in air was thenbubbled upwards through the slurry at a rate of 200 m³/h under a slurryagitation of 240 rpm. During the carbonation, the temperature of thereaction mix was controlled and maintained between 20-25° C. After 85min (calculated from start of introduction of said gas) the introductionof gas was stopped. Immediately after carbonation, the resultingsuspension was wet-ground in a vertical bead mill at a flow rate of 320liters/h, resulting in a specific grinding energy consumption of about100 kWh/dry ton. To the resulting suspension, 1.8 kg of FormamidineSulfinic Acid (DegaFAS® from Degussa-Hüls) was added. The slurry wasthen transferred to a pressurized vessel and heated to about 130° C. for30 min. The product was recovered as an aqueous suspension.Characteristics and physical properties are given in column B of table1.

Example 2

The following illustrative example of the invention involves thepreparation of hydromagnesite by calcining and slaking white dolomitestone and contacting the obtained magnesium hydroxide with gaseous CO₂to convert the obtained magnesium hydroxide into hydromagnesite, whereinthe carbonation is carried out with a starting temperature of about 12°C. and a subsequent ageing step.

White Dolomite stones (Hammerfall A/S) were crushed to yield a grainsize of 10-50 mm and calcined in a rotary kiln at 1050° C. for 60 min.The resulting burned Dolomite (CaO.MgO) was ground in a ball mill toobtain a powder with mean particle size of about 40 μm (CILAS laserdiffraction method). 200 kg of said burned Dolomite were slaked byadding to 1 000 liters of 50° C. tap water in a stirred reactor. Theburned Dolomite was slaked for 30 min under continuous stirring and theresulting suspension was adjusted to about 8% solids content viadilution with water. The carbonation was conducted in a 1 800 literbaffled cylindrical stainless steel reactor equipped with a gasingagitator, a stainless steel carbonation tube to direct a carbondioxide/air gas stream to the impeller and probes for monitoring the pHand conductivity of the suspension. 1 800 liters of the suspensionobtained in the slaking step was adjusted to a temperature of 12° C. andadded to the carbonating reactor. A gas of 26% by volume of CO2 in airwas then bubbled upwards through the slurry at a rate of 200 m³/h undera slurry agitation of 240 rpm. During the carbonation, the temperatureof the reaction mix was not controlled and allowed to rise due to heatgenerated in the exothermic reaction. After 85 min (calculated fromstart of introduction of said gas) the introduction of gas was stopped.The suspension was then transferred to a pressurized vessel and heatedto about 130° C. for 30 min. The product was recovered as an aqueousslurry. Characteristics and physical properties are given in column C oftable 1.

TABLE 1 physical data column example A comparative B 1 C 2 specificsurface area BET m²/g 42.6 34.4 55.9 PSD avg. particle size d₅₀ CILAS920 μm 14.34 4.9 3.39 brightness (DIN 53140) R457 (ISO 2469) % 82.9 89.993.3 yellow index (DIN 6167) 7.4 0.5 1.1 solids content % 9.4 9.7 7.8viscosity (Brookfield 100 rpm) mPas 30 1600 780 pH Slurry 7.3 9.4 10.2mineralogical composition XRD Hydromagnesite Mg₅(CO₃)₄(OH)₂•4(H₂O) %90.8 98.3 35.7 Nesquehonite Mg(HCO₃)(OH)•2(H₂O) % Calcite CaCO₃ % 1.11.6 58.2 Dolomite CaMg[CO₃]₂ % Brucite Mg(OH)₂ % 3.1 Periclase MgO % 50.1

As can be gathered from the data shown in table 1, the inventive methodespecially leads to products having a significantly lower particle size(average particle size d₅₀).

The invention claimed is:
 1. A process for preparing hydromagnesitecomprising the steps of: a) providing at least one magnesium oxidesource; b) providing gaseous CO₂ and/or carbonate-comprising anions; c)slaking the magnesium oxide source of step a) in an aqueous environmentto convert the magnesium oxide at least partially into magnesiumhydroxide; d) contacting the magnesium hydroxide of step c) with saidgaseous CO₂ and/or carbonate-comprising anions of step b) to convert themagnesium hydroxide at least partially into precipitated nesquehonite;e) grinding the precipitated nesquehonite from step d); and f) treatingthe ground precipitated nesquehonite of step e) in a heat-ageing step toform hydromagnesite.
 2. The process according to claim 1, wherein the atleast one magnesium oxide source is selected from the group consistingof magnesium oxide, magnesite, dolomite, huntite, magnesium carbonate,magnesium hydroxide, brucite and mixtures thereof.
 3. The processaccording to claim 1, wherein the gaseous CO₂ comes from an external CO₂supply or from the recirculation of CO₂ or both.
 4. The processaccording to claim 1, wherein the carbonate-comprising anions areselected from the group consisting of sodium carbonate, potassiumcarbonate, sodium hydrogen carbonate, potassium hydrogen carbonate andmixtures thereof.
 5. The process according to claim 1, wherein thestarting temperature of step d) is adjusted to a temperature of between5° C. and 35° C.
 6. The process according to claim 1, wherein thestarting temperature of step d) is adjusted to a temperature of between10° C. and 30° C.
 7. The process according to claim 1, wherein theheat-ageing step e) is carried out at a temperature of at least 90° C.8. The process according to claim 1, wherein the heat-ageing step e) iscarried out at a temperature of between 90° C. and 150° C.
 9. Theprocess according to claim 1, wherein the heat-ageing step e) is carriedout at a temperature of between 110° C. and 140° C.
 10. The processaccording to claim 1, wherein the heat-ageing step e) is carried out ata temperature of between 120° C. to 135° C.
 11. The process according toclaim 1, wherein the heat-ageing step e) is carried out at a temperatureof about 130° C.
 12. The process according to claim 1, wherein theheat-ageing step is carried out for a period of time of 20 min to 60min.
 13. The process according to claim 1, wherein the heat-ageing stepis carried out for a period of time of 20 min to 40 min.
 14. The processaccording to claim 1, wherein the heat-ageing step is carried out for aperiod of time of 20 min to 35 min.
 15. The process according to claim1, wherein the nesquehonite obtained after the grinding comprisesparticles of which at least 50% by weight have an average particle sizeof less than 25 μm.
 16. The process according to claim 1, wherein thenesquehonite obtained after the grinding comprises particles of which atleast 50% by weight have an average particle size of less than 20 μm.17. The process according to claim 1, wherein the nesquehonite obtainedafter the grinding comprises particles of which at least 50% by weighthave an average particle size of less than 15 μm.
 18. The processaccording to claim 1, wherein the nesquehonite obtained after thegrinding comprises particles of which at least 50% by weight have anaverage particle size of less than 10 μm.
 19. The process according toclaim 1, wherein the hydromagnesite obtained by the process has aspecific BET surface area of 10 m²/g to 150 m²/g.
 20. The processaccording to claim 1, wherein the hydromagnesite obtained by the processhas a specific BET surface area of 10 m²/g to 100 m²/g.
 21. The processaccording to claim 1, wherein the hydromagnesite obtained by the processhas a specific BET surface area of 20 m²/g to 70 m²/g.
 22. The processaccording to claim 1, wherein the hydromagnesite obtained by the processhas a degree of whiteness of at least 80%.
 23. The process according toclaim 1, wherein the hydromagnesite obtained by the process has a degreeof whiteness of at least 85%.
 24. The process according to claim 1,wherein the hydromagnesite obtained by the process has a degree ofwhiteness of between 85 and 99%.
 25. The process according to claim 24,wherein the hydromagnesite obtained by the process comprises particleshaving an average particle size d50 of less than 20 μm.
 26. The processaccording to claim 24, wherein the hydromagnesite obtained by theprocess comprises particles having an average particle size d50 of lessthan 15 μm.
 27. The process according to claim 24, wherein thehydromagnesite obtained by the process comprises particles having anaverage particle size d50 of less than 10 μm.
 28. The process accordingto claim 24, wherein the hydromagnesite obtained by the processcomprises particles having an average particle size d50 of less than 5μm.
 29. The process according to claim 24, wherein the hydromagnesiteobtained by the process is further treated with one or more fatty acids.30. The process according to claim 29, wherein the one or more fattyacids is selected from the group consisting of stearic acid, palmiticacid, behenic acid, montanic acid, capric acid, lauric acid, myristicacid, isostearic acid and cerotic acid.